Conventionally, an optical modulator having an optical waveguide structure in which an optical waveguide is formed on a substrate has been known. A Mach-Zehnder interference type optical modulator including a plurality of optical waveguides has a configuration in which light waves propagating through different optical waveguides are combined, and the intensity of light after combining is determined by the difference in the amount of phase change between light waves propagating through the optical waveguides (for example, refer to Patent Literature No. 1). By changing the difference in the amount of phase change, it is possible to change the output intensity of the optical modulator.